1. Field of the Invention
The present invention relates to wireless communication devices, and more particularly, to a wireless communication device using an adaptive array antenna to perform wireless communication.
2. Description of the Related Art
Development of digital cellular wireless communication systems using DS-CDMA (Direct Spread Code Division Multiple Access) technology has been actively pursued since such systems are considered next-generation mobile communication systems enabling wireless multimedia communications.
In ordinary wireless communication systems, an adaptive array antenna system has been introduced in anticipation of increase in the subscriber capacity or expansion of the cell radius. The adaptive array antenna system is a system wherein a plurality of antennas are arrayed so that their directivities can be dynamically varied in response to changes in the electromagnetic environment.
FIG. 7 illustrates a schematic configuration of an adaptive array antenna system. The illustrated adaptive array antenna system is a model showing a basic receiving operation thereof and only the elements necessary for explaining the system are shown in the figure.
The adaptive array antenna system 10 includes antenna branches 10-1 to 10-4 and a data demodulator 1c. The antenna branches 10-1 to 10-4 respectively comprise antennas 1-1 to 1-4, analog amplifiers 10b-1 to 10b-4, and A/D converters 10c-1 to 10c-4. The data demodulator 1c comprises multipliers 11-1 to 11-4, an adder 12, and a weighting factor setting unit 1c-1.
The antennas 1-1 to 1-4 receive radio signals, which are then amplified by the analog amplifiers 10b-1 to 10b-4, respectively, and the A/D converters 10c-1 to 10c-4 convert the respective amplified analog signals to digital signals d1 to d4 (other elements in the individual branches, such as frequency converters, are omitted from the figure).
The weighting factor setting unit 1c-1 determines weighting factors in accordance with setting information A, and the multipliers 11-1 to 11-4 respectively multiply the output signals d1 to d4 from the A/D converters 10c-1 to 10c-4 by the weighting factors W1 to W4 set by the weighting factor setting unit 1c-1. The adder 12 adds up the four weighted signals and outputs the resulting signal.
Suppose the antennas 1-1 to 1-4 receive a radio signal with an arrival angle φ. In this case, if the phase plane is set using the antenna 1-1 as a reference antenna, the antennas 1-2 to 1-4 receive the radio signal with phase differences λ, 2λ, and 3λ, respectively, because of path differences (the phase difference between adjacent antennas is λ).
For the multiplier 11-2 connected to the antenna 1-2, the weighting factor setting unit 1c-1 generates a weighting factor W2 for rotating the phase by λ and sends the factor W2 to the multiplier 11-2, so that the multiplier 11-2 multiplies the signal d2 by the weighting factor W2 to correct the phase difference λ. Also, for the multiplier 11-3 connected to the antenna 1-3, the weighting factor setting unit 1c-1 generates a weighting factor W3 for rotating the phase by 2λ and sends the factor W3 to the multiplier 11-3. Thus, the multiplier 11-3 multiplies the signal d3 by the weighting factor W3 to correct the phase difference 2λ. Further, for the multiplier 11-4 connected to the antenna 1-4, the weighting factor setting unit 1c-1 generates a weighting factor W4 for rotating the phase by 3λ and sends the factor W4 to the multiplier 11-4, whereupon the multiplier 11-4 multiplies the signal d4 by the weighting factor W4 to correct the phase difference 3λ.
The multiplications using the respective weighting factors make it possible to cancel out all phase differences λ between the antennas 1-1 to 1-4, allowing the multipliers 11-1 to 11-4 to output signals which are in phase with each other. The adder 12 adds up the in-phase signals, so that signal reception with high gain can be performed.
After reaching the antennas 1-1 to 1-4, the radio signal is subjected to analog amplification, A/D conversion, etc., as mentioned above. Nonlinear circuit elements for performing these functions have individually different characteristics, and the characteristics also vary in response to changes in temperature or other environmental conditions as well as with the lapse of time. Therefore, the phase characteristics (phase rotations) of the antenna branches 10-1 to 10-4 are not exactly the same but differ from one to another.
Consequently, the antenna branches 10-1 to 10-4 involve their own phase shifts, and if these phase shifts are not removed, the phase differences cannot be completely canceled out, making it impossible to perform the in-phase addition with accuracy.
It is therefore necessary to carry out calibration for detecting and removing variations in the phase characteristic among the antenna branches in the system. A circuit for performing such calibration is called calibrator. The calibrator corrects the phase shifts of the individual antenna branches, whereby the antenna outputs can be weighted with accuracy in the data demodulator 1c. 
As conventional adaptive array antenna techniques, a technique has been proposed in which beamforming is carried out to generate delay profiles for a plurality of beams and a path is detected based on the delay profiles, whereby the scale of circuitry is minimized even in cases where the number of antenna elements is increased (e.g., Unexamined Japanese Patent Publication No. 2003-283404 (paragraph nos. [0022] to [0029], FIG. 1)).
In adaptive array antenna systems, beamforming is carried out with the beam directivity of the array antenna adaptively controlled such that a narrow beam is directed to a desired station for communication.
When signal is transmitted from a base station equipped with an adaptive array antenna to a terminal, the base station carries out DL (Down Link) beamforming in such a manner that the beam directivity is highest in the DoA (Direction of Arrival) which is estimated from the UL (Up Link) signal transmitted from the terminal. A circuit for estimating the arrival direction (arrival angle) at the time of DL beamforming is called adaptive array searcher.
Thus, adaptive array antenna systems are provided with the calibrator function and the adaptive array searcher function. In conventional systems, however, the calibrator circuit and the adaptive array searcher circuit are mounted separately, though they include equivalent functional blocks that can be shared, and the equivalent circuit elements are operated at the same time, giving rise to the problem that the scale of the circuitry as well as the power consumption increase.